1. Field of the Invention
The present invention relates to improvements to hydraulic pumps with pistons and controlled suction valves.
2. Description of the Prior Art
Generally, hydraulic pumps with pistons are comprised of a plurality of pistons reciprocated by means of a cam. The cam can be connected to the driving shaft, the pistons sliding in cylinders formed in the pump body, or the cam being fixed and the pistons sliding in cylinders formed in a rotary barrel driven by the driving shaft.
In these types of pumps, it is known to use hollow pistons through which the pumped hydraulic liquid flows, the hydraulic liquid penetrating inside the piston via the head of the latter and exiting therefrom at its other end. In such a case, the pistons rest on an inclined face of the cam via pads sliding on the surface of the cam, the pads being formed with a central orifice extending through them and coming to ride, during their suction stroke, on a lunula engraved on the surface of said cam. When the pad thus slides above the lunula, a fluid communication is established between the chamber in which moves the cam and the inside of the corresponding hollow piston via the lunula, the central orifice of the sliding pad and the piston head, also formed with a central orifice extending through it. This communication is interrupted when the pad leaves the lunula and slides on the smooth face of the cam. The liquid contained inside the piston and compressed by the motion of the latter is then discharged via a spring loaded valve located at the bottom of the cylinder.
The cam may be of any appropriate shape, for example an eccentric, whereby the pistons are radial, or a slanting plate, whereby the pistons are then parallel to the pump driving shaft.
Pumps of these types have two disadvantages. The first disadvantage is caused by the fact that the rotation direction of the pump driving shaft must always be the same, since otherwise suction and discharge cannot take place. In order to remedy this disadvantage, French Pat. No. 67/18715, dated June 17, 1977, has proposed to place on the front face of the slanting plate, without lunula, a plate pivotable over 180.degree., the plate being in turn provided with a lunula. Thus, by simply rotating the plate on the front face of the slanting plate it is possible to reverse the suction and discharge phases and therefore to reverse the pump rotation direction. Unfortunately, this rotation of the plate on the slanting face cannot take place automatically and requires manual intervention.
The second disadvantage arises from the fact that since the hydraulic liquids have non-negligible compressibility at high pressures (200 bars and above), it is necessary to angularly offset the beginning of the lunula with respect to the line of steepest slope of the slanting plate face in order to avoid the hydraulic fluid admission phase beginning before the hydraulic liquid inside the piston has finished expanding. If such a precaution is not taken, the liquid under pressure is discharged in the suction circuit, insted of the discharge circuit thereby lowering the pump efficiency. The degree of the offset is a function of the pump rotation speed and of the compressibility rate of the fluid transported. The latter depends on the pressure, temperature, quantity of air dissolved in the fluid, etc. Thus, it should be necessary to adapt the offset to the variations of the various variables. It has not, however, been known how to provide pumps in which the angular offset of the lunula can be modified at will. Therefore, a medium value of the offset is therefore determined, which is a compromise between the extreme values of the pump operation parameters. The result is that the pump operates in an optimum manner only under very aleatory conditions.
In order to mitigate these disadvantages, the applicant has replaced the feeding system by means of a lunula and sliding pads by suction valves.
In order that a pump equipped with suction valves (instead of a lunula such as hereabove described) may operate correctly and rotate at a high speed, the suction valves must return very quickly to their closed position. Thus are avoided phenomena of rebound, poor closing and interference with the discharge phase of the pump.
In order that the suction valves return quickly and correctly to their closed position, it is necessary that they be pushed by strong springs. But then, during the suction phase, the piston must overcome the strong effort applied to the suction valve. Generally, its suction power is not compatible with the valve return effort. The suction does not take place and the pump does not operate. Even if a compromised return-suction power is reached, the suction power of the piston is affected, at least of the value of the suction valve load. Therefore, the pump loses a major portion of its volumetric performance.
To solve this difficulty, the applicant has disclosed in U.S. Pat. No. 4,486,152 which has been incorporated by reference herein, a slanting plate pump having for each piston an admission valve subjected to the action of two antagonistic springs: a first spring pushing it in the closing direction and a second spring pushing it in the opening direction. The second spring is associated with a cam rotatably connected with the pump driving shaft so that the force exerted by the second spring becomes substantially equal to that exerted in the reverse direction by the first spring at the beginning of the suction phase. The application also discloses a cam formed such as to be placed in two positions offset by 180.degree. with respect to one another, thereby allowing reversing of the pump rotation direction.
This arrangement operates satisfactorily, but its disadvantage is that it is rather complex and therefore costly, and the present invention relates to a simplified device that achieves the samd advantages.
In accordance with an illustrative embodiment demonstrating features and advantages of the present invention, there is provided a pump having at least a piston, each piston being associated with a suction valve, the valves being returned to their closing position by a spring, wherein the return spring of the suction valve is associated on the one hand with the valve and on the other hand with a push-piece cyclically actuated by a cam driven by the pump driving shaft, in such manner that the action of the spring on the valve is cancelled at the beginning of the suction phase and reestablished at the end of the suction phase.